Types of DNA

• Wilkinson at all define the double helical structure of DNA and their measuring by the X-rya crystallography.
• This type of DNA known as B-DNA and it is found in many species.
• Other than this A.C,D and Z DNA also found in organism.

According to the helical structure and morphology DNA are following type.

1. Circular DNA molecules :

• This type of DNA molecules are commonly occurred in bacteriophages, prokaryotes, mitochondria and chloroplast. 
• They may be single stranded, or double stranded.
• This type of DNA molecule is without 3′ and 5′ free ends, therefore it is regarded as circular DNA. 
• Usually the circular DNA molecule is not associated with histone proteins but it also occurs in compact form. like linear DNA molecule.
• In some prokaryotes double stranded circular DNA molecule forms large loops in such a way that each loop represents part of a circle
• Such type of DNA are called supercoiled DNA. 
• A molecule without coiling is called relaxed DNA .

2. Linear DNA Molecules :

• This type of DNA is found in the nucleus of Eukaryotes and in some viruses. 
• It is a long thread like structure with two free ends, i.e., 3′ end and 5′ end. 
• In the form of chromosome, this DNA is found to be tightly associated with proteins in highly condensed form. 
• Although viral DNA molecules are linear in shape but they are not tightly associated with proteins.
• In them protein is found as an envelope in which DNA is packed.
• The linear DNA also found in different structural 

2. Linear DNA Molecules :

• This type of DNA is found in the nucleus of Eukaryotes and in some viruses. 
• It is a long thread like structure with two free ends, i.e., 3′ end and 5′ end. 
• In the form of chromosome, this DNA is found to be tightly associated with proteins in highly condensed form. 
• Although viral DNA molecules are linear in shape but they are not tightly associated with proteins.
• In them protein is found as an envelope in which DNA is packed.
• The linear DNA also found in different structural 

 Different forms of DNA double helix (A, B, C and Z-DNA)

Right handed DNA

1. 1. ‘A’-DNA

• This is right handed DNA.
• In this type of DNA, the length of helix is 28.15 Å 
• There are 11 base pairs per helix. 
• The diameter of the helix (20.2) with the axial rise per base pair is about 2.36 Å. 

2. B’-DNA or Right handed DNA

• The structural model for this type of DNA was proposed by Watson and Crick. 
• It is a most common form of DNA. 
• The helix is right handed with the length of about 34 Å. 
• Each turn contains 10 pairs of mononucleotide units. 
• The rise of helix per base pair is 3.4 Å and the tilt of base pair is 6.3. 
• The diameter of DNA molecule is 20 A. 
• The repeating unit is a mononucleotide and the orientation of sugar molecule is not alternating. 
• Each successive base pair in the stack turns 36° 

3. ‘C’-DNA and ‘D’-DNA

• Both types of DNAS are right handed double helical structures, 
• similar to ‘B’-DNA with minor differences.
• In ‘C’ type of DNA, the size of helix is greater than found in A type of DNA but smaller than found in B-DNA. 
• It is about 31 Á. There are 9.33 base pairs per turn of the helix. 
• There is an axial rise of about 3.32 Ă and the tilt of base pairs is about 7.8. 
• However D-DNA has only 8 base pairs. 
• It lacks guanine.

4. ‘Z-DNA or Left handed DNA

• Rich and his co-workers have proposed a left handed double helical model for DNA molecules. 
• In this model zig-zag sugar phosphate backbones run in antiparallel organization 
• This is known as ‘Z’-DNA. In this model the sugar residues have alternating orientation , 
• Its one coil is 4.66 nm long having 12 nucleotide base pairs.
• The distance between two base pairs is 0.375nm .
• In this type of DNA the sugar residues show alternating orientation therefore the repeating unit is a dinucleotide.